United States Patent [191 Green

United States Patent [191 Green 1541 MULTIFREQUENCY BROADBAND POLARIZED HORN ANTENNA [76] Inventor: Robert A. Frosch, Administrator of the National Aeronautics and Space Administration, with respect to an invention of Kenneth A. Green, N. Andover, Mass. 1211 Appl. No.: 8,209 [22] Filed: Jan. 31,1979 1511 Int. Cl.3... HOlQ 13/00 [52] U.S. Cl.... 343/786; 343/755; 343/772; 343/781 R [58] Field of Search... 343/729, 755, 776, 781, 343/786, 854,772 1561 References Cited U.S. PATENT DOCUMENTS 3,369,197 2/1968 Giger et a].. 343/786 3,568,204 3/1971 Blaisdell... 343/786 3,754,273 7/1974 Takeichi et al.. 343/786 3,936,838 2/1976 Foldes... 343/786 3,949,404 4/1976 Green... 343/761 OTHER PUBLICATIONS Green, Kenneth, Multifrequency, Broadband, Dual- Polarized Antenna, NASA Tech. Brief, Winter 1976, NPO 13866, p. 516. Takeda et al., Broadbanding of Corrugated Conical Horns, IEEE Trans. on Ant. and Prop., Nov. 1976, pp. 786-792. Kumazawa et al., Wide-Band Communication Satellite c111 4,258,366 [451 Mar. 24, 198% Antenna, IEEE Trans on Ant. and Prop., May 1975, pp. 404-407. Primary Examiner-David K. Moore Attorney, Agent, or Firm-Monte F. Mott; John R. Manning; Paul F. McCaul P I ABSTRACT A multifrequency, broadband, dual-polarized corrugated conical horn antenna is simultaneously fed a multiplicity of signals, two for each of five frequencies, with each of a pair of signals fed in each of two orthogonal planes for excitation of a desired spherical hybrid mode (HE1 1). The lowest frequency is fed into the horn through orthogonal pairs of colinear slots, each pair being fed by coaxial tee power dividers. Other signals are fed through a circular waveguide connected to the vertex. Band reject cavities block the next higher frequency from passing through the low frequency feed slots. The highest frequency signals are fed through orthogonal ports near the far end of the circular waveguide. The intermediate frequency signals are fed through orthogonal ports spaced along the waveguide. Filtering is incorporated for each to maintain isolation and low insertion loss, a quarterwave step transformer is used between the highest frequency (37 GHz) ports and the two next lower frequencies (21 GHz and 18 GHz) to provide a short circuit for these two lower frequencies, and a TM11 mode generator for the highest frequency is used as a short circuit for the next lower frequency (10.69 Ghz). 4 Claims, 9 Drawing Figures

U.S. Patent Mar. 24, 1981 Sheet 1 of 7 4,258,366 FIG.l 6.633 833 GHz FIG. 2

U.S. Patent Mar. 24, 1981 Sheet 2 of 7 4,258,366 FIG.3a 12 16.3 b

U.S. Patent Mar. 24, 1981 Sheet 3 of 7 4,25 8,3 66

U.S. Patent Mar. 24, 1981 Sheet 4 of 7 4,25 8,366 12 24 36 40 60

U.S. Patent Mar. 24, 1981 Sheet 5 of 7 4,258,366 4% 36 24 12 +-Angle-+ 12 24 36 48 Degrees FIG.6

U.S. Patent Mar. 24, 1981 Sheet 6 of 7 4,258,366 48 36 24 12 -Angle- 12 24 Degrees FIG.7 36 48

U.S. Patent Mar. 24, 1981 Sheet 7 of 7 4,25 8,366 48 36 24 12 -Angle4 12 24 36 48 Degrees FIG.8

MULTIFREQUENCY BROADBAND POLARIZED WORN ANTENNA ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457). BACKGROUND OF THE INVENTION This invention relates to a multifrequency, broadband, dual-polarized or circularly polarized horn antenna. Many applications require a highly efficient, multifrequency dual-polarized scanning antenna system. Ideally, the system should have the same constant width, symmetrical beam for both polarizations and all fre- quencies, and should have very low side lobes at all frequencies. A horn-reflector antenna system in disclosed is US. Pat. No. 3,949,404 capable of producing a spherical aperture phase front using a hyperbolic reflector illuminated by a corrugated conical horn. The farfield beam produced has low sidelobes and high efficiency. This antenna system is insensitive to frequency and polarization changes, and would therefore be suitable for multifrequency dual-polarized scanning purposes, but the problem is simultaneously feeding the corrugated conical horn with more than one frequency when the frequencies are spaced over one or more octaves. Dual-frequency horn antennas have been devised in the past for widely spaced frequencies. The higher frequency is fed at the vertex of the horn in the usual manner, and the lower frequencies are fed into the horn where the cross-sectional dimensions are greater. See for example Hirayuki Kumazawa, Masaki Kayama and Yashio Kataoka, Wide-Band Communication Satellite Antenna Using a Multifrequency Primary Horn, IEEE Transactions on Antennas and Propagation, May 1975, pp. 404-407. A problem with this approach is the limited number of widely spaced frequencies which can be accommodated with good isolation of the frequency channels. Thus, while such a multi-frequency antenna system may be adequate for the particular application for which designed, it would not be able to produce as many as five or more concentric distinct beams with similar width; and high efficiency over a wide range (e.g., 6.6 to 37 GHz). Such a requirement would be, for example, a scanning multichannel microwave radiometer (SMMR) to be used on the Numbus G and Seasat A satellites. The SMMR requires a highly efficient multifrequency antenna system to be achieved by using a corrugated conical horn (CCH) and scanning reflector with geometry similar to that disclosed in U.S. Pat. No. 3,949,404. The reflector is an offset paraboloid of 31-4,258,366 inch diameter projected aperture fabricated out of graphite-epoxy, and the feed subassembly is a dual PO- 60 larized horn. The design of the horn is an extension of a - previous multi-frequency ring loaded CCH reported in a National Aeronautics and Space Administration Tech Brief NPO 13866 published in Winter 1976 NASA Tech Briefs at page 516. The horn is a broadband, 65 dual-polarized antenna. The ring-loaded CCH has been reported by Fumio Takeda and Tsutomu Hashimoto, Broadbanding of Corrugated Conical Horns, IEEE 5 10 15 20 25 30 35 Trans on Ant. & Prop., Nov. 1976, pp. 786-792. See also U.S. Pat. No. 3,754,273 granted to the authors as coinventors with Yoshihiro Takeichi. The problem of adapting a ring loaded CCH antenna as a feedhorn in an SMMR, or other application requiring a highly efficient, multifrequency dual polarized antenna, is to obtain operation on both polarizations at a multiplicity (typically 5) of frequencies over a wide range (typically from 6 to 37 GHz) to provide a multiplicity of signals (two for each frequency, one for each polarization) simultaneously, with low insertion loss and good isolation between frequencies and polarizations, as well as extremely low sidelobes and symmetrical nearly equal beamwidths for all polarizations and frequencies. It is that problem that is solved by this invention. SUMMARY OF THE INVENTION In accordance with the present invention, a multifrequency, broadband, dual-polarized corrugated conical horn is provided with longitudinal slots near the small diameter end of the cone, one pair in each of two orthogonal planes for excitation in a desired spherical hybrid mode (HEII) at the lowest of the frequencies. Paired slots are fed by coax-to-waveguide adapters and each symmetrical pair is fed through a transmission line power divider, such as a coaxial tee power divider. Other higher frequencies feed through a circular waveguide connected to the small diameter end of the horn, with the highest frequency fed at the end of the waveguide farthest from the smallest diameter end, and intermediate frequencies at spaced points between the remote end of the waveguide and the of the small diameter end of the conical horn antenna. At each point, two orthogonal ports are provided for excitation of the desired orthogonal polarizations at the same frequencies. Each of the ports is fed through a waveguide which contains a low pass filter to reject frequencies above their own. and a auarterwave step transformer is pro- 40 vided between thehighest frequency port at the end-and the first pair of ports along the circular waveguide to provide a short circuit for the next one or more lower frequencies. Each intermediate frequency along the circular waveguide is thus isolated from signals of 45 higher frequency by low pass filters. The ports in the side of the cone fed at the lowest frequency are isolated from the next higher frequency by a band reject cavity set for the next higher frequency. The circular waveguide is provided with sections of minimum diameter 50 for propagation of the frequencies fed through the ports in the sections. A tapered TMI 1 mode generator for the highest frequency is used as a short circuit for the next lower frequency. The novel features that are considered characteristic 55 of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings. BRIEF DESCRiPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating the geome- - - try of an exemplary antenna system utilizing 2 multifrequency corrugated conical horn (CCH) that is the subject of the present invention. FIG. 2 is a cross section of the multifrequency corru- gated conical horn of FIG. 1. I FIGS. 3a and 36 are front and rear perspective views of the corrugated conical horn of FIG. 1.

4,258,366 3 4 FIGS. 4, 5, 6, 7 and 8 illustrate the E and H plane gated conical horn 12 at 6.633, 10.69, 18, 21 and 37 beam patterns which are typical of each polarization at GHz. This yields primary illumination patterns with each of the different frequencies fed into the CCH of low sidelobes and approximately equal E and H plane FIG. 1. beamwidths for all frequencies and polarizations. 5 The HE11 spherical hybrid mode for the lowest fre- DESCRIPTION OF PREFERRED quency is excited into the corrugated conical horn EMBODIMENTS through side ports 160 and 16b provided in the form of a Referring now to the drawings, FIG. 1 shows the pair of longitudinal slots 170 and 176 near the vertex of geometry of a highly efficient multifrequency antenna the cone. Each slot is backed by a coax-to-waveguide system comprised of an offset paraboloid reflector 10 10 adapter, 18a and Ub, and each symmetrical pair is fed a fed by a corrugated conical horn (CCH) 12 with a mul- colinear signal in phase through a coaxial tee power tiplicity of frequencies. The reflector has a 31-inch di- divider 19 (FIG. 34. This arrangement yields symmeameter projected aperture and is fabricated of graphite- try in each plane, for suppression of undesirable higher epoxy. The CCH is of a broadband, multimode ring- order modes, and excellent isolation between orthcgoloaded design with ports on the side for excitation with 15 nal polarizations. The coax-to-waveguide adapters and one low frequency, and a circular waveguide 14 con- coaxial tee power divider for the orthogonal polarizanected to the vertex (small diameter end) of the horn for tion are identified in FIGS. 3a and 3b by the reference excitation with higher frequencies as will be described numerals lw, and 19. The two coaxial connectors (one more fully with reference to FIGS. 2 and 3. for each polarization) are identified by the reference Although reference is made to excitation or feed- 20 numerals 20 and 20. ing of the antenna system, it should be noted that in The desirable features of a corrugated horn are: equal this exemplary embodiment, the multifrequency an- E and H plane beamwidths; low sidelobes; coincident E tenna is designed for use a as multichannel microwave and H plane phase center of radiation; near-constant radiometer. As such the system is intended to receive beamwidth and phase center over a broad band. These rather than transmit, but as in all passive microwave 25 features are obtained if a sufficient length of capacitive systems, the performance is reciprocal. Therefore refer- surface inside the horn is produced by the corrugations. ence will be made to feeding the antenna system with Fundamentally, each annular groove is a short circuited a multiplicity of frequencies for convenience only, it stub which will produce a capacitive surface effect being understood that the antenna system may as well when electrically equal to or greater than a quarter receive the same frequencies simultaneously. 30 wave and less than a half wave deep. (This is also true The dual-polarized frequencies fed are, for example, between 3 of a wave and one wavelength, etc.) The five frequencies from 6.6 GHz to 37 GHz. The system effective bandwidth in the fundamental range then aprequires simultaneous operation on orthogonal polarizations at 6.633, 10.69, 18, 21 and 37 GHz with low inserpears to be limited to less than an octave, as in a rectangular waveguide. However, use of impedance steps tion loss and near optimum illumination on all 10 chan- 35 (ridges or rings) can increase the bandwidth to greater nels. As just noted above, both polarizations of the than 3:1, just as is done in ridged waveguides. By proper lowest of the five frequencies are coupled into the CCH choice of groove configuration and spacing, it is possivia ports on the side of the cone while both polariza- ble to obtain capacitive surface impedance inside the tions of the highest frequency are fed in at the end of the horn at all five frequencies needed over the 6:l band. It waveguide which functions as a multiplexer for cou- 40 should be noted that many other groups of frequencies pling both polarizations of four widely spaced frequen- could be utilized over even greater bandwidths, alcies into the vertex of the cone. The primary patterns though the entire band may not be usable simultaachieved yield highly efficient secondary patterns of all neously, and some unique combinations may not be 10 signals simultaneously, two (dual polarized) signals obtainable. The inoperative (inductive) frequency reat each frequency. As will be pointed out with reference 45 gions may be reduced or eliminated by tapering the to FIG. 2,good isolation between frequencies and PO- corrugation size along the horn, or interlacing different larizations is maintained. The reflector 10 is designed as a section of a parabosizes. The remaining four dual-polarized frequencies, 10.69, loid to subtend an angle of 40 +42 =82, with edge 18, 21 and 37 GHz, are fed through the circular waveillumination of approximately -18 db. The scan axis 50 guide 14 into the apex of the horn 12. The selected for the particular and exemplary application of the an- diameter and symmetry of the circular waveguide for tenna system is perpendicular to Earth s surface and the propagation of 10.69 inhibits the propagation of higher beam follows a circular arc of k25 on the Earth s order modes until about 21 GHz. To avoid undesired surface, as the reflector scans about the vertical axis. modes at 37 GHz, the dual polarized signals introduced The CCH (12) and the feed waveguide 14 are stationary 55 at that frequency through ports 21 and 21 at the end of at the focal point (the vertex of the horn 12) with its axis the waveguide 14 must be concentrically fed through a coincident with the horn axis and the scan axis. step transformer 26. Further a TMI 1 mode is generated Referring now to FIG. 2, it shows all of the ports in in a region 22 in the waveguide 14 where its diameter a single cross section of the assembly necessary for five changes, and this mode is phased with the TEI 1 mode to colinear frequencies of one polarization. A similar struc- 60 produce a cosine tapered E field at the end of the circuture in an orthogonal plane excites all of the ports neces- lar waveguide (vertex of the conical horn). This means sary for the five co-linear frequencies of a polarization that the end of the waveguide becomes a low sidelobe orthogonal to the one polarization. Consequently, it radiating aperture at a point in the large conical horn should be clearly understood that FIG. 2 illustrates the which is the phase center of the spherical HE1 1 modes 10-port feed for the horn 12 which operates with dual 65 of the other frequencies. It can be seen that the size of polarization at each of five frequencies simultaneously this aperture determines the beamwidth at this frein a 6:l band. In that manner, orthogonally polarized quency, while the flare angle of the horn is the controlspherical hybrid modes (HE1 I) are excited in the corru- ling parameter at all other frequencies. These must be

4,258,366 5-6 - compatible, but the waveguide size is also limited by the not be as good due to the ripple on the peak of the other frequencies it must propagate. This thus presents beams at21 and 37 GHz. At 21 GHz the ripple is due to limitations on the choice of frequencies which may be' higheqorder HE modes generated at the horn and cirmore severe than the limits of the corrugations. Qf cular wavegui n. At 37 GHz the ripples are course, a quad-ridged circular or square waveguide 5 caused by the turbing the dual mode aperture would increase the bandwidth, potential but would also pattem introduce other moding considerations describe atterns of the horn feeding an Chen and Tsandoulas in "Modes of Quadruple Ri offset paraboloid have been recorded on the five an- Waveguides," IEEE Transactions on MTT Aug. tenna subsytems built. Identical, excellent results were u.801. 10 obtained. In all cases the far-field beamwidths and side- The 8-ports coupling into the circular waveguide 14 is termed an ''orthomode multiplexer." It is comprised of the &port assembly of orthomode transducers and filters used as a low loss combiner for both polarizations of four frequencies into the circular waveguide. FIG. 2 15 shows how colinear polarizations are combined. 37 GHz enters at the end and fed past the 18 GHz, 21 GHz and 10.69 GHz ports 23, 24 and 25, which contain respective low pass, waffle-iron type filters 23u, 24u and,251 to reject frequencies above their own. The quarter- 20 wave step transformer 26 between 37 GHz and 18 GHz ports is a short circuit for 18 GHz and 21 GHz, and the 37 GHz TM11 mode generator 22 is the short circuit for 10.69 GHz. The 37 GHz port needs no filter as it is the highest frequency and its waveguide 21a is a high-pass 25 filter. This concept is used at all ports to isolate substantially lower frequencies. However, the low pass filter 23n in the 18 GHz port does not isolate 21 GHz, as this is not a large enough frequency separation for a low pass application. Other type filters could be used (e.g. 30 band-pass or band-reject) but the lowest insertion loss technique that could be used was found to be positioning the 18 GHz port between the 21 GHz port and its short circuit 26, at a null of the 21 GHz standing wave. It should be noted that to isolate widely separated 35 frequencies could require rejection of several modes, and that a short circuit at the coupling aperture is necessary to avoid exciting modes in the common waveguide. A waffle-iron filter rejects all modes, and produces a short at a common reference plane. The pres- 40 ence of the ring loaded corrugations in the CCH effectively isolates 18 GHz, 21 GHz and 37 GHz from the 6.633 GHz ports, but additional isolation at 10.69 GHz (fundamental mode only) is provided by a single band reject cavity in each waveguide identified by the refer- 45 ence numeral 2& and 286 in FIG. 2. In that manner, insertion loss is low and good isolation between frequencies and dual polarizations is maintained for five separate frequencies with high effwiency and constant beam width for all frequencies as shown by FIGS. 4, 5, 50 6,7 and 8 which compare the beam patterns for the dual polarizations at the different frequencies. For each frequency side lobes for each polarization are very low and the beamwidth is symmetrical for both polarizations. 55 The primary E and H plane radiation patterns at all five frequencies have thus been demonstrated to be very nearly the same. Cross polarization level is less than about - 20 db in each case. The E and H plane symmetry is excellent at each frequency. The - 10 db beam- 60 width is 50" minimum and 60" maximum. (A constant - 10 db width within the space available was a design goal.) A longer horn would improve the frequency independency. Also, changes in the 10.69 GHz coupling ports would improve the cross polarization, and hence, 65 isolation, at that frequency. A design goal of constant -3 db widths could be used for purposes other than illuminating a secondary aperture, but the results would lobes measured agreed with the values to be expected of an optimum focused horn with the same primary pattern. This is evidence that the phase centers of radiation for all ten beams are within a fraction of wavelength of each other. In summary a unique, complex and highly successful antenna system has been disclosed particularly suited for satellite-borne radiometers. However, the multifrequency, dual polarization performance could also have wide application in radar and communication systems because of its excellent characteristics. Specifically, it has a constant beamwidth, high efficiency and dual polarization for operation over an octave band, or at several frequencies spaced over several octaves. This is achieved through the combination of a ridged corrugated horn, coupling apertures through the corrugations, a dual mode aperture at the horn vertex, and a dual polarized (orthomode) multiplexer feeding through the dual mode aperture. The multiplexer employs lowpass filters, orthomode transducers and a unique spacing of ports at one frequency to obtain isolation without resonant elements. Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art. For example, the pair of ports at any frequency can be simultaneously fed by an equal power divider such that the relative phase at the ports is go", and circular polarization of the beam results. Such polarization flexibility for any or all frequencies may be useful in some applications. As a further example, feeding a pair of orthogonal ports at a frequency with relative phase at the ports of 0", and varying the amplitude sinusoidally such that the modulation is 90" out of phase, rotating linear polarization of the beam results. I claim: 1. A multifrequency, broadband horn antenna com- prising a ring loaded corrugated conical horn having a large diameter end anda small diameter end, said small diameter end having a common axis with said large diameter end and said conical horn being open at the large diameter end thereof, and having at least one diametrically opposed pair of ports at a position near the small diameter end for feeding a colinear signal of low frequency, and a multiplexer for simultaneously feeding additional colinear frequency signals spaced over multiple octaves into said conical horn through said small diameter end, said multiplexer comprising a waveguide having a plurality of ports, and means for fixing standing waves of signals fed into said waveguide, at least one port for each additional colinear frequency signal, said ports being spaced along said waveguide with the port for the highest frequency signal at the end furthest from said small diameter end, and the ports for the intermediate frequency

7' 4,250,366 signals spaced between the end of the waveguide section by a TMll mode generator for the highest freand the small diameter end, each intermediate co- quency being fed through orthogonal ports at the ends linear signal port having connected thereto a low of a third section of said waveguide of a diameter suitpass filter to reject frequencies above the frequency able for propagation of said highest frequency, said fed into the port through the fdter, and at least one 5 third section being connected to said second section by port for a lower frequency than the highest being at a quarter wave step transformer to provide a short a position corresponding to the standing wave null circuit for each frequency fed into said second section, of a higher frequency for isolation from signals of thereby to prevent lower frequency signals from propathe higher frequency. gating into said third section. 2. The combination of claim 1 wherein said multi- 10 4. The combination of claim 3 wherein two frequenplexer has a first section next to the small diameter end of said conical horn for propagation of the highest frequency fed into said multiplexer through orthogonal ports at the end of the section remote from the small cies are fed into said section with a frequency separation between the two frequencies not large enough for the low pass filter of the ports of the frequency of the lower of the two frequencies to isolate the other of the two diameter end of said conical horn. 3. The combination of claim 2 wherein said waveguide has a second section of a diameter suitable for propagation of at least one higher frequency fed into said multiplexer through orthogonal ports in the second 15 frequencies, said ports for said lower of the two frequencies being between the ports of the higher of the two frequencies and said quarter wave step transformer and being located at a null of the standing wave of the higher of the two frequencies. section, said second section being connected to the first 20 * * * * * 8 25 30 35 40 45 50 55 65